In a rack assist type electric power steering apparatus, the rotation of an electric motor is slowed down by a reduction gear mechanism comprising a pulley-belt mechanism and thereafter converted into the axial movement of a rack shaft, for example, through a ball screw mechanism surrounding the rack shaft (see, for example, Japanese Examined Patent Publication No. 4(1992)-28583).
The abovementioned pulley-belt mechanism comprises a small pulley disposed on the same axis line with an output shaft of the electric motor, and a large pulley surrounding the rack shaft.
A reduction gear mechanism of such a kind of electric power steering apparatus is required to be small-sized and at the same time to obtain a high reduction gear ratio.
However, if the small pulley is made small-diameter for this purpose, the radius of curvature of a belt region wound on the small pulley becomes small and it is feared that the life of belt is shortened because of its flexion fatigue. Besides, it is feared that torque transmission loss is caused by increased inner friction at curved portions of the belt.
On the contrary, if the large pulley is made large-diameter, the whole apparatus becomes large thus to lessen its mountability on a vehicle.
Therefore, an object of the present invention is to provide an electric power steering apparatus which is small-sized and suitably mountable on a vehicle, and can achieve a high reduction gear ratio and high durability.